1. Field of the Invention
The present invention relates to a tool holder, and more specifically, to a tool holder for mounting a cutting tool such as drills, end mills, etc. to a main spindle of a machine tool.
2. Description of Prior Art
Hitherto, a tool holder as shown in FIG. 5 has been offered. This tool holder is configured as a retracting type, and comprises a main body 1, a spring collet 10, an intermediate rod 15, a clamp bolt 20 and a pull stud 30. At the axial center portion of the main body 1, a center hole 4 that passes through the main body 1 from the top end to the rear end is formed. The collet 10 is mounted to the front tapered hole portion 4a of the center hole 4 and pressure-holds a shank portion 40a of the cutting tool 40 by being pulled to the rear side (in the arrow "A" direction). The front portion of the rod 15 is connected to the rear portion of the collet 10, and to the rear portion, the clamp bolt 20 is screwed.
In this tool holder, rotating the clamp bolt 20 in the arrow "a" direction (forward direction) retracts the collet 10 via the rod 15 and gives the collet 10 a tightening force to the cutting tool 40. In order to remove the tool 40, the clamp bolt 20 is rotated in the opposite direction to arrow "a", which cancels the tightening force of the collet 10. To rotate the clamp bolt 20, the pull stud 30 is removed and a hex key (not illustrated) is fitted to a hexagon hole 22 of the bolt head 21, or the pull stud 30 is kept mounted and the hex key is inserted in the through hole 31 to be fitted to the hexagon hole 22.
Now, in the tool holder comprising as above, the torque for rotating the clamp bolt 20 in the reverse direction to untighten the tool (to push out the collet) is larger than the torque for rotating the clamp bolt 20 in the forward direction to tighten the tool (to retract the collet). This is because biting phenomena occur between the collet tapered portion 10a and the main body tapered hole portion 4a when the collet 10 is retracted by rotating the clamp bolt 20 in the forward direction.
Consequently, even rotating the clamp bolt 20 in the reverse direction to remove the tool is unable to release the connection between the tapered portion 10a and the tapered hole portion 4a, and to release this connection, an impact must be given to the head 21 of the clamp bolt 20 from the rear to tap out the collet 10. However, this has a problem of poor operability.
Therefore, there has been proposed a method wherein the tip end surface 30a of the pull stud 30 is located adjacent to the head 21 of the clamp bolt 20, and a hex key is inserted in the through hole 31 with the pull stud 30 mounted to the main body 1 to rotate the clamp bolt 20 in the reverse direction (see Japanese Laid Open Patent Publication No. 8-150504). In this method, rotating the clamp bolt 20 in the reverse direction causes the head to come in pressure-contact with the tip-end surface 30a of the pull stud 30 and presses forward the collet 10, and releases the connection between the tapered portion 10a and tapered hole portion 4a.
However, in this method, at the time of untightening the collet 10, the head portion 21 comes in pressure-contact with the pull stud tip end surface 30a by surface friction, requiring a very large torque to rotate the clamp bolt 20 in the reverse direction and creating a problem of difficult operation.